Creating via contacts with ohmic behavior between layers of metal in a semiconductor device can be difficult to achieve because there will typically be a layer of native oxide present between the metal layers. Ohmic behavior is achieved with there is a linear relationship between a current flowing through the contact and a voltage drop across the contact. The native oxides of metals, such as aluminum oxide for aluminum or copper oxide for copper, tend to be insulators. The presence of the insulating layer can prevent the via contacts from having true ohmic behavior.
With reference now to FIG. 1, there is shown a diagram illustrating a cross sectional view of a portion of a prior art semiconductor device 100. The semiconductor device 100 includes a substrate 105 with metal layer 110 formed above the substrate 105. The substrate 105 may contain other structures that are not shown in FIG. 1. Formed above the metal layer 110 can be a layer of titanium nitride (TiN) 115 and a layer of an oxide material 120. The TiN layer 115 and the oxide layer 120 can form an antireflective coating (ARC).
The presence of the ARC can be specific to the semiconductor device 100 and may not be present in general. For example, in applications where light scattering from the surface of the semiconductor device 100 can cause a problem with image quality, the ARC may be created to help reduce the scattering of the light incident on the surface of the semiconductor device 100. In an application where scattered light is not a concern, it may not be necessary to create the ARC.
The TiN layer 115 and the oxide layer 120 may be etched to form an opening 122 for a via 130. The TiN layer 115 and the oxide layer 120 can then be covered by a spacer layer 125 that may be created from a photoresist material. The spacer layer 125 can then be patterned and the photoresist material can be removed with a photoresist develop process to form the via 130. The via 130 can then be filled with a metal, for example.
The etch of the TiN layer 115 and the oxide layer 120 down to the metal layer 110 would typically require two separate etch processes, and once the metal layer 110 is exposed, an oxide of the metal used in the metal layer 110 can form over the exposed portions of the metal layer 110. An expanded view of the semiconductor device 100 illustrates an oxide layer 135 formed between the metal layer 110 and the via 130 and the spacer layer 125 and the via 130.
With reference now to FIGS. 2a and 2b, there are shown diagrams illustrating an electron micrograph of a cross section of a prior art semiconductor device and a data plot of current versus voltage for typical via contacts formed as shown in FIG. 1. The diagram shown in FIG. 2a illustrates a via formed over a metal layer. The diagram shown in FIG. 2b illustrates a data plot of current versus voltage for exemplary via contacts. If the via contacts exhibited true ohmic behavior, the amperes versus volts plots would be linear. The amperes versus volts plots shown in FIG. 2b are clearly non-linear. Therefore, the via contacts do not exhibit ohmic behavior.
A technique that can be used to help create ohmic via contacts is to use a sputter etch to etch the metal layer, such as the metal layer 110, to help remove any oxide layer that may be covering the metal layer 110 prior to the creation of the via contact. With a sputter etch, the metal layer 110 is subject to bombardment by high energy ions to remove a portion of the metal layer. The high energy ions will also remove any oxide layer along with part of the metal layer.
One disadvantage of the prior art is that the use of the sputter etch requires an additional fabrication process step, which can increase the complexity of the fabrication process. The increased complexity can increase the fabrication costs as well as potentially decrease the yield of the fabrication process.